In laparoscopy, for example, a camera is usually coupled to the laparoscope. The operator moves the instrument and the laparoscope down through entrance ports, so-called trocars, placed in the abdominal wall. In order to enable the operator to use both hands, an assistant/cameraman is required to hold and guide the laparoscope/camera as the operator desires. Savings can be made if an operation assistant can be replaced by a mechanical structure which can hold the medical instrument, e.g. the laparoscope, in a predetermined position and which can be simply repositioned by the operator himself. Various devices are known which can be used for this purpose, but all are complicated by having motor-driven functions. Such a known device is shown and described in U.S. Pat. No. 5,351,676. Here there are achieved the functions described by way of introduction with the aid of telescoping or articulated arms with electric motors and electromagnetically controlled braking means for adjusting and fixing in a certain position. This known device is particularly expensive and complicated.